The present invention relates to a hydraulic control system for a lock-up clutch of a hydrodynamic device of an automatic transmission for a vehicle.
In the case of automatic transmissions having a hydrodynamic device, such as a torque converter, with a lock-up clutch, the lock-up clutch is kept engaged when the vehicle speed is higher than a predetermined value during operation with a predetermined gear ratio or speed ratio. One of such hydraulic control systems is described in U.S. Pat. No. 4,369,865 issued Jan. 25, 1983 to Sunohara et al. The known hydraulic control systems of the above kind pose the following problems. The first problem is in that since the lock-up clutch is left engaged as long as the above mentioned travelling condition is satisfied even if the vehicle is coasting, the vibrations from the engine is transmitted to the drive system and thus the driveablity is degraded. The second problem is in that the engine revolution speed during coasting operation is elevated when the lock-up clutch is engaged as compared to the case when the lock-up clutch is not engaged and thus fuel economy during coasting operation is poor as compared to the latter case. The engine revolution speed is elevated because there is no slip when the lock-up clutch is engaged during coasting operation.